The use of nitrogen fixing bacteria to promote the growth of leguminous plants is known. Bacteria of the rhizobia genera are used for such purposes and various strains have been isolated from the root nodules of selected leguminous plant varieties and from the soils in which such varieties are grown and are available as cultures in the marketplace for use in inoculating leguminous plants of the varieties thus selected. These cultures are available as fresh cultures of the bacteria in nutrient solutions, as concentrated and frozen cultures in nutrient solutions, and as cultures on agar.
In practice, there are two commonly used methods of introducing the bacteria to the soil. One method follows procedures which involve coating the legume or seed with material containing the viable bacteria so that the seed and bacteria are simultaneously introduced to the soil during the process of planting the legume or seed. The cultures in this case are usually premixed with a suitable solid carrier such as humus, sawdust, clay or other suitable particulate material by the supplier of the bacteria. Thereafter the farmer at the time of planting mixes the bacteria impregnated carrier and seed to be used in planting the crop in predetermined proportions and under conditions such that the seed becomes coated with the carrier. The coated seed is then transferred to the planting apparatuses and delivered to the soil.
The coated seed methods have certain disadvantages. For one, there is a need in many planting areas to protect the seed from fungus and/or insects during the period of germination. This is usually done by coating the seed with a suitable fungicide and/or insecticide and which are toxic to the bacteria. This fact, of course, precludes the use of the seed coating methods for introducing the bacteria to the soil in those farming areas where the need arises to use pesticides for the protection of the seed.
Yet another disadvantage to this method of introducing the bacteria to the soil can be attributed to the type apparatus which is used for planting the seeds and which require in most instances that the seed be substantially dry so as to avoid agglomeration and clogging of the seed drills. The bacteria on the other hand are sensitive to direct sunlight and elevated temperature conditions and require a viable moist and nutrient environment both prior to and following introduction to the soil. As a consequence it not infrequently happens that the bacteria is killed prior to its introduction to the soil because the seed dries out in the bulk containers that are used in the planting procedures or is alternatively subjected to such high temperatures in such containers as to effectively kill the bacteria. This problem with elevated temperatures is especially apparent in the southern regions of the United States.
The other common method for introducing the bacteria to the soil avoids the problems that are associated with the use of pesticides for coating the seeds. In this case the cultures are premixed with a suitable solid carrier of particulate material which is frequently formed into pellets and which is thereafter delivered to the soil by apparatus separate and apart from that used for delivering the seed to the soil. The devices used for delivering the bacteria to the soil in this instance also require that the particulate material be substantially dry so as to avoid clogging and this, of course, frequently leads to a high temperature and dry environment in the bulk containers used for handling the particulate material in the planting area and which, of course, is detrimental to the survival of the bacteria during the planting procedures.